A variety of implantable infusion devices are available for treating patients. For example, implantable infusion devices are used for delivering therapeutic substances to a target location of a patient. The implantable infusion devices are implanted subcutaneously in a convenient location in the patient. An infusion catheter is connected to an outlet of the device and positioned in the patient to allow delivery to the target location. A therapeutic substance may then be introduced percutaneously into a reservoir of the implanted device by inserting a needle into a port assembly of the device and delivering a fluid containing the therapeutic substance to the device via the needle.
Because the device is implanted within the patient and cannot be seen directly, care must be taken to monitor the withdrawal and filling of the therapeutic substance into the reservoir. For example, when removing a drug from the reservoir it is advantageous to know when the all or substantially all of the drug has been removed. Moreover, it is additionally advantageous to know when the reservoir has been filled with the new drug. Commercially available sensors that indicate the amount of fluid in the reservoir are not ideal due to size and space limitations. Such sensors may include a float connected to a variable resistor, a pressure sensor, sometimes connected to a mercury manometer, or low voltage capacitors where the fluid can go between them to register a reading.
A need therefore exists for a system capable of detecting the flow of therapeutic substance out of and into the reservoir of an implantable delivery device. A need also exists for indicating a reasonable approximation of how full the reservoir is during filling emptying and pumping procedures.